The most common method for determining the most popular plant parameters is to use a specially configured, hand-held light detector sensitive to the sun. The user carries such a hand-held device through a forest after having gathered reference data in a clear-to-the-sky field, and the solar radiation transmitted through the canopy is recorded.
One disadvantage of such a method comes in separating effects generated by leaves from effects generated by branches, since both leaves and branches block light. Moreover, changing sun and cloud conditions limit the time in which valid data can be taken before it becomes necessary to take new reference data in the clear-to the sky field.
Aircraft-based instruments using the sun have been used to study vegetation; however, such instruments have atmospheric and sun angle effects to confuse the results. The present inventor has used an aircraft-based detector with a flash lamp to take data, but the backscatter from the earth's surface convolves both soil and leaf undercover and living plant characteristics. The present inventor's experiences with aircraft-based detectors are described in the papers "An Active Optical Remote Sensing System for Vegetation Index Determination" by J. E. Kalshoven, et al and "Airborne Ground Illuminator for Hyperspectral Validation and Calibration" by J. E. Kalshoven, presented at conferences of the IEEE and SPIE, respectively, in July, 1996. Both of these papers are hereby incorporated by reference in their entireties into the present disclosure.
Fluorescence-based systems use lasers to excite a surface. Such systems must be custom-designed for specific applications in laboratory settings.